The objective of this project is to find the key to achieve ultra-highly efficient (η_<ext>>40%) light emitting diodes (LEDs) covering ultraviolet (UV), blue, green and white spectral range. The results obtained in the period of 2000 to 2001 can be summarized as follows.(1) Time-resolved electroluminescence (TREL) was performed in green, blue and near-UV LEDs based on In_xGa_<1-x>N single quantum well structures under pulsed current injection, by which emission mechanism could be assessed in the actual device operation. It was found that EL linewidth as well as Stokes shift was enhanced with increasing mean In composition in active layers due to the increase of exciton (and/or carrier) localization.(2) Time-space resolved photoluminescence (TSRPL) spectroscopy was set up by focusing UV-pulsed laser (wavelength : 266 nm, pulse width : 1.5 ps) in an optical microscope using UV optical components. By using this system, epitaxially lateral overgrown (ELO) GaN was assessed, and site selective photo-excitation was made to either 4 μm-wide window region with threading dislocation density (TDD) of 10^8 cm^<-3>, or 8 μm-wide wing region with TDD of 10^6 cm^<-3>. As a result, PL lifetime in the wing region was slightly larger than that in the window region, indicating that threading dislocations act as nonradiative centers. However, the difference of lifetime was very small considering two-orders difference in TDD.Therefore, it is suggested that pathway to nonradiative recombination is limited by other centers whose origin is probably point defects.(3) TSRPL in air-bridged laterally epitaxial grown (ABLEG) InGaN layers revealed that the addition of small amount of In to active layers results in the enhancement of PL efficiency because of the reduction of nonradiative recombination centers originating from point defects